Question

II am trying to understand Scala collections by adding a new collection as follows:

class NewColl[V](values:Vector[V],someOtherParams)
extends IndexedSeq[V] with IndexedSeqLike[V, NewColl[V]] {

  def fromSeq[V](seq: Seq[V]): NewColl[V] = ...

  override def newBuilder[V]: Builder[V, NewColl[V]] =
    new ArrayBuffer[V] mapResult fromSeq[V]
}

but I get the following error:

overriding method newBuilder in trait TraversableLike
   of type => scala.collection.mutable.Builder[V,NewColl[V]];
method newBuilder in trait GenericTraversableTemplate
   of type => scala.collection.mutable.Builder[V,IndexedSeq[V]] has incompatible type

Any Idea?

Answer 1

What I do in similar cases is to look at what the standard library does in similar cases. Looking at concrete subclasses of IndexedSeq they seem to mix in GenericTraversableTemplate. With that in mind, reworking your code to use it I get:

import collection.mutable._
import collection.generic.GenericTraversableTemplate
import collection.generic.GenericCompanion

class NewColl[V](values:Vector[V]) extends IndexedSeq[V] with 
    GenericTraversableTemplate[V, NewColl] {

  def fromSeq[V](seq: Seq[V]): NewColl[V] = new NewColl(Vector(seq: _*))

  override def companion: GenericCompanion[NewColl] = new GenericCompanion[NewColl]() {
    def newBuilder[A]: Builder[A, NewColl[A]] = new Builder[A, NewColl[A]] {
      val elems = new ArrayBuffer[A]()
      def +=(a:A) = { elems += a; this } 
      def clear() { elems.clear }
      def result(): NewColl[A] = fromSeq(elems)
    }
  }

}

(with someOtherParams removed for clarity)

Note that there are other questions on SO related to building classes on the scala 2.8 collection framework. For instance 5200505 which points to one of my favorite document The Architecture of Scala Collections. Also recently, Josh Suereth wrote a blog entry on creating your own collection class.